Motor operated valve combinations or valves are commonly used in nuclear and other power plants to perform routine opening and closing of water and steam lines and, more importantly, for those valves designated as safety related, to perform such openings or closings to assure the prompt, safe shutdown of the plant under accident or other emergency or abnormal operating conditions. For those valves of this class whose safety function is to close a water, steam or other line, it is important that the valve achieve full or complete closure when called upon to do so. Most motor operated valves in use today are controlled by a torque switch which shuts off or stops the operation of the operator motor when a predetermined valve actuator torque limit is reached and a spring pack or other component moves in response to actuator torque in excess of the limit to trip or actuate the torque switch. Since the torque switch displacement is approximately proportional to the actuator torque above some preload torque, and since the actuator torque is approximately proportional to the stem force, the torque switch displacement set point can be and generally is used to turn off the motor at approximately the valve stem force required to fully close the valve. A certain amount of stem thrust is also required for a motor operated valve (MOV) to close against differential pressure. The required thrust can be calculated as the sum of the packing friction force, the stem rejection force, and the differential pressure force. The packing friction force is the force required to push the value stem through the packing which has been compressed against the stem to prevent leakage. The stem rejection force is the internal pressure force trying to spit out or reject the valve stem. It is equal to the internal line pressure times the cross sectional area of the stem. The differential pressure force is the friction force opposing the inward movement of the valve disc as it is pushed in by the valve stem to close off the opening of the valve. It is equal to the differential pressure across the valve times the effective area of the valve disc times the coefficient of friction of the valve disc against the downstream seat, this coefficient of friction is often referred to as the valve factor.
A control switch, typically the torque switch, is adjusted to trip and thereby turn off the motor at a stem thrust level which is generally higher than the computed required closure thrust so as to provide a comfortable thrust margin. A thrust verification test must be run at the valve to determine the actual thrust applied to the valve stem at control switch trip (CST), making sure the actual thrust exceeds the computed required closure thrust by a comfortable margin. The at-the-valve test is repeated periodically to insure that degradations have not reduced the level of the thrust being developed at CST. The most likely such valve degradation which may occur is lubrication degradation, which increases the coefficient of friction between the threads of the valve stem and the threads of the stem nut that drives the valve stem causing the motor actuator to develop less stem thrust for the given amount of torque at the established torque switch trip setting. Between each at-the-valve periodic thrust verification test, maintenance personnel typically have no way of knowing whether lubrication degradation has occurred, and if it has, by how much it may have reduced the stem thrust developed at the established CST.
The present invention couples results from a specified remote test with results of a previous at-the-valve thrust verification test, to provide a method for determining the degree of lubrication degradation which has occurred, and for determining the resulting new thrust margin which must be evaluated for sufficiency to ensure valve closure. The present invention combines the results of two tests of a motor operated valve separated by a period of time sufficient for lubrication degradation to have possibly occurred. In the first test performed at the valve, both a stem thrust vs time trace and a motor power vs time trace are concurrently developed as the closing valve seats. In the second test, which may be performed at a location remote from the valve, only the motor power vs time trace is developed as the closing valve seats. The motor power trace can be developed from remotely located sensors preferably at the motor control center.
Lubrication degradation is determined from the change, first test to second test, in the time rate of motor power build-up between the onset of valve wedging and torque switch trip. The thrust margin, defined as the increase in stem thrust between the onset of wedging and torque switch trip, is determined from the time between the onset of valve wedging and torque switch trip, first test or second test, multiplied by the time rate of thrust build-up, as determined from the first test, after the onset of wedging. As will be discussed, the time rate of thrust build-up after the onset of wedging is not significantly affected by lubrication degradation, or by the rate of thrust build-up before the onset of wedging, sometimes referred to as the rate of loading or load sensitive behavior.